Add thread/unarchive to restore archived rollouts (#9843)

## Summary
- Adds a new `thread/unarchive` RPC to move archived thread rollouts
back into the active `sessions/` tree.

## What changed
- **Protocol**
  - Adds `thread/unarchive` request/response types and wiring.
- **Server**
  - Implements `thread_unarchive` in the app server.
  - Validates the archived rollout path and thread ID.
- Restores the rollout to `sessions/YYYY/MM/DD/...` based on the rollout
filename timestamp.
- **Core**
- Adds `find_archived_thread_path_by_id_str` helper for archived
rollouts.
- **Docs**
  - Documents the new RPC and usage example.
- **Tests**
  - Adds an end-to-end server test that:
    1) starts a thread,
    2) archives it,
    3) unarchives it,
    4) asserts the file is restored to `sessions/`.

## How to use
```json
{ "method": "thread/unarchive", "id": 24, "params": { "threadId": "<thread-id>" } }
```

## Author Codex Session

`codex resume 019bf158-54b6-7960-a696-9d85df7e1bc1` (soon I'll make this
kind of session UUID forkable by anyone with the right
`session_object_storage_url` line in their config, but for now just
pasting it here for my reference)

codex-rs/core/tests/suite/rollout_list_find.rs

@@ -3,14 +3,15 @@ use std::io::Write;
 use std::path::Path;
 use std::path::PathBuf;
 
+use codex_core::find_archived_thread_path_by_id_str;
 use codex_core::find_thread_path_by_id_str;
 use tempfile::TempDir;
 use uuid::Uuid;
 
-/// Create sessions/YYYY/MM/DD and write a minimal rollout file containing the
+/// Create <subdir>/YYYY/MM/DD and write a minimal rollout file containing the
 /// provided conversation id in the SessionMeta line. Returns the absolute path.
-fn write_minimal_rollout_with_id(codex_home: &Path, id: Uuid) -> PathBuf {
-    let sessions = codex_home.join("sessions/2024/01/01");
+fn write_minimal_rollout_with_id_in_subdir(codex_home: &Path, subdir: &str, id: Uuid) -> PathBuf {
+    let sessions = codex_home.join(subdir).join("2024/01/01");
     std::fs::create_dir_all(&sessions).unwrap();
 
     let file = sessions.join(format!("rollout-2024-01-01T00-00-00-{id}.jsonl"));
@@ -37,6 +38,12 @@ fn write_minimal_rollout_with_id(codex_home: &Path, id: Uuid) -> PathBuf {
     file
 }
 
+/// Create sessions/YYYY/MM/DD and write a minimal rollout file containing the
+/// provided conversation id in the SessionMeta line. Returns the absolute path.
+fn write_minimal_rollout_with_id(codex_home: &Path, id: Uuid) -> PathBuf {
+    write_minimal_rollout_with_id_in_subdir(codex_home, "sessions", id)
+}
+
 #[tokio::test]
 async fn find_locates_rollout_file_by_id() {
     let home = TempDir::new().unwrap();
@@ -79,3 +86,16 @@ async fn find_ignores_granular_gitignore_rules() {
 
     assert_eq!(found, Some(expected));
 }
+
+#[tokio::test]
+async fn find_archived_locates_rollout_file_by_id() {
+    let home = TempDir::new().unwrap();
+    let id = Uuid::new_v4();
+    let expected = write_minimal_rollout_with_id_in_subdir(home.path(), "archived_sessions", id);
+
+    let found = find_archived_thread_path_by_id_str(home.path(), &id.to_string())
+        .await
+        .unwrap();
+
+    assert_eq!(found, Some(expected));
+}